The present invention relates generally to the field of indicator devices which, upon the occurrence of a predefined condition in an electric circuit, automatically display a visual indication that the condition has occurred. Circuit breakers, for example, display a colored flag if the electric current through a circuit exceeds a safe level. Similarly, fuse state indicators display a visual indication of whether a fuse has blown.
Some fuse state indicators ("FSIs") indicate fuse blow by turning on a light emitting diode ("LED") or other illuminating indicator. Others indicate fuse blow by changing the position of a mechanical indicator.
Fuse State Indicators having LED indicators require an electric current to illuminate the LED. Since this current is typically supplied by the fused circuit, such LED indicators require current to continue flowing through the fused circuit even after the fuse blows. If power to the circuit is removed to allow replacement of the blown fuse, the indicator turns off, eliminating any indication that the fuse has blown. Further, if an LED indicator is used with high voltage fuses, the LED itself must be protected, thereby increasing the complexity and cost of the FSI while adversely affecting its reliability.
FSIs having a mechanical indicator include a fusible wire which retains a plunger or other mechanical indicator in a non-indicating position. Upon fuse blow, current flows through the fusible wire causing it to heat up and melt. Upon melting, the wire releases a spring which pushes the mechanical indicator to an indicating position. Unlike LED indicators, such devices require no current to provide a visual indication and therefore maintain the indication if electric power is removed from the fused circuit. However, such devices are not reusable. Once the fusible wire melts, the indicator must be replaced with one having an intact fusible wire.
In general, the object of the invention is to provide an improved indicator device. More specifically, one object of the invention is to provide a non-volatile indicator device which, after an event has occurred, requires no electric power to maintain a visual indication that the event has occurred. Another object is to provide a mechanism for manually resetting the indicator to allow the indicator to be re-used. Still another object of the invention is to provide an improved fuse state indicator for detecting when a fuse blows and promptly providing a visual indication of the condition of the fuse without requiring any electric current to maintain that indication.